The present invention relates to a step-down constant-current transformer, including a controllable switch downstream in outgoing circuit from the one input and connected in series to a choke, and a capacitor connected in parallel at the output, of the species defined in the first part claim 1.
A step-down constant-current transformer including a controllable switch downstream from the one input, connected in series to a choke inductor, and a capacitor connected in parallel at the output, is known from the book xe2x80x9cHalbleiter-Schaltungstechnikxe2x80x9d (Semiconductor-Circuit Engineering) chapter 18.6.1, pp. 563-566, by U. Tietze and Ch. Schenck, Springer-Verlag publishers, 9, ed. 1989, a free-wheeling diode being provided in parallel, upstream in incoming circuit from the choke. The step-down constant-current transformer is used for converting an input d.c. voltage into another, lower output d.c. voltage. Due to its simple construction and high level of efficiency, this transformer is preferably used in motor vehicles. One difficulty with these known step-down constant-current transformers is the so-called gap interrupted operation, when the output current is smaller than half of the average load current.
This problem occurs particularly in the case of step-down constant-current transformers, where the parallel-connected free-wheeling diode is replaced by a controllable switch to increase the efficiency level and to form a synchronous rectifier. In the case of transformers having synchronous rectifiers, reverse currents occur occasionally, which flow from the output in the direction of the input, through the choke, and also discharge the output capacitor. As such, this causes the efficiency level to deteriorate and the output ripple to increase. Such an arrangement is known to the applicant as internal related art within the company, but cannot be verified in print.
To avoid reverse currents caused by the synchronous rectifier, it is possible to switch off the synchronous rectifier during reverse current flow. However, for this purpose, it is necessary to detect the current direction. The current direction can be conventionally detected by measuring the current, e.g. using a shunt or a current transformer. However, in this case, a reduction of the output signal range may be unavoidable, since the measuring amplifier indicates the positive and the negative current range. This is particularly disadvantageous for circuits that are operated using a unipolar voltage supply in motor vehicles, for example.
The object of the circuit configuration in accordance with the present invention is to reliably and inexpensively detect current direction without the stated disadvantages and to prevent reverse currents in step-down constant-current transformers.
The step-down constant-current transformer configured in accordance with the present invention has the advantage of ensuring the detection of an occurrence of a reverse current using simple means and of creating the opportunity to prevent or stop the flow of this reverse current.
According to the present invention, this is principally achieved in that, instead of the free-wheeling diode for forming a xe2x80x9csynchronous rectifierxe2x80x9d connected in parallel, upstream from the choke, a second controllable switch is provided; both controllable switches are made of MOSFET transistors; a control unit, in particular a pulse-width control unit, is provided for the MOSFET transistors; the voltage across the parallel-connected MOSFET transistor is monitored to determine whether a reverse current, i.e., a current flowing through the choke from the output in the direction of the input of the constant-current transformer, or, as the case may be, the electric potential at the center tap of the half-bridge formed by the two MOSFET transistors is detected; and in that in the event of such a reverse current, the parallel-connected MOSFET transistor is blocked to prevent the reverse current.
As a result of measures set forth in the additional claims, preferred further refinements and embodiments, as well as improvements of the step-down constant-current transformer recited in claim 1 are possible.
In a particularly effective embodiment of the present invention, for detecting voltage, a comparator is provided that compares the electric potentials at the drain terminal and source terminal of the parallel MOSFET transistor to one another and then produces a controlling output signal when the electric potential is higher at the drain terminal than at the source terminal, which is preferably at zero potential.
According to an advantageous further refinement of this practical embodiment of the present invention, an enabling logic is provided for the synchronous rectifier by which the output signal of the comparator is combined with the output signal from the control unit intended for the gate of the MOSFET transistor arranged in parallel. In a simple embodiment, the enabling logic is an AND circuit to which the gate control signal for the gate of the parallel MOSFET transistor and the output signal of the comparator are supplied as input signals.
In an additional advantageous embodiment, an operational amplifier or a comparing element is provided as the comparator to which, at the inverting input, the electric potential at the drain terminal of the parallel MOSFET transistor, and, at the non-inverting input, the electric potential of the source terminal at zero potential of the parallel MOSFET transistor is supplied.
In an advantageous embodiment and improvement of this exemplary embodiment of the present invention, the output signal of the comparator is stored in a storage element, in particular a microcontroller, and is also made available for additional application purposes. As such, diagnostic purposes, for example, can be satisfied and system conditions can be determined.
In an additional improvement of this exemplary embodiment, a capacitor between the positive input and the zero potential is provided in parallel upstream from the controllable, series-connected MOSFET transistor. As a result of this input capacitor, the input d.c. voltage is smoothed and overvoltage protection is achieved.
The constant-current transformer configured in accordance with the present invention is particularly advantageously used in a parallel connection of a plurality of power modules. Corresponding to a further particularly advantageous application, the constant-current transformer is used for forming interleaved buck converters, where the individual power modules are controlled in a phase-displaced manner.